Dimethyl carbonate (DMC) is an important intermediate widely used in the chemical industry. Due to its low toxicity, DMC is considered a green reagent with many promising application prospects. At room temperature, DMC is a colorless, flammable liquid. It is mutually soluble with many organic solvents, such as alcohol, ester and ketones, and also having good hydrophobicity. Since DMC contains highly reactive functional groups, e.g., methyl, methoxy, carbonyl and methoxy carbonyl groups in its molecular structure, it is a versatile chemical and can be used as a better substitute for dimethyl sulphate, phosgene, or methyl halide, which are toxic and/or corrosive, in a variety of synthetic organic routes such as methylation, carbonylation, and transesterification, which meets the increasing demands for clean and green processes in the chemical industry.
DMC can also be used as a solvent to replace halogenated solvents. In addition, DMC has a high octane number and high oxygen content (53% wt), and as such can be used as a good additive to transportation fuels to reduce harmful emissions.
Currently, DMC can be produced on industrial scale via the following processes: 1) phosgene; 2) oxidative carbonylation of methanol; 3) transesterification; and 4) urea alcoholysis.
In phosgene route, DMC is produced from methanol and phosgene in concentrated NaOH solution. Because of the use of phosgene, which is highly toxic (chemical weapons reagent and potentially used as weapons of mass destruction) and corrosive, this process has been phased out in the industry.
Non-phosgene routes include oxidative carbonylation of methanol in liquid phase (see U.S. Pat. No. 4,318,862 to Romano et al.) and gas phase (see U.S. Pat. No. 5,162,563 to Nishihira et al.). Whether the process is carried out in liquid phase or gas phase, the main catalysts are metal chlorides, such as CuCl2 or CuCl, which loses its activity rapidly. The deactivated catalyst reacts with water, which is a product of the oxidative carbonylation process, and forms hydrochloric acid. Hydrochloric acid is highly corrosive to reactor vessels, which results in high capital investment. The chloride ions in the product stream is difficult to remove, thus negatively impacts the quality of the final product. Additionally, the process operates at elevated pressures and other stringent conditions, making it economically less viable.
The transesterification route produces DMC by transesterification of cyclic carbonate (e.g., ethylene carbonate) with methanol (see U.S. Pat. No. 4,661,609 to Knifton, U.S. Pat. No. 4,691,041 to Duranleau et al.). The main disadvantage of the route includes cyclic carbonate, being an expensive chemical, is used as feedstock. Alternatively, cyclic carbonate may be obtained via epoxide reacting with CO2. However, epoxide is also an expensive feedstock. The slow reaction rate of epoxide with CO2, the need of high reaction pressure, and the thermal equilibrium limitation of the reaction further hamper such alternative. In addition, such process leads to the significant co-production of alkylene glycol, which puts constraints on the scale of DMC production.
The urea alcoholysis route, being based on inexpensive and renewable raw materials, is an attractive alternative. In this route, DMC is produced by reacting primary aliphatic alcohols such as methanol with urea in the presence of various heterogeneous and homogeneous catalysts such as dibutyl tin dimethoxide, tetraphenyl tin, etc (see, e.g., P. Ball et al, Synthesis of carbonates and polycarbonates by reaction of urea with hydroxyl compounds. C1 Mol. Chem. 1984, 1, 95). However, the route is not without limitations, such as low yield and high energy cost. Ryu et al. revealed the process of producing DMC from methanol and urea in a distillation tower (U.S. Pat. No. 5,902,894 to Ryu and U.S. Pat. No. 6,392,078 to Ryu et al.). Unfortunately, the conversion rate and selectivity are rather low due to catalysts and process conditions utilized. More recently, homogeneous and heterogeneous catalysts and catalytic distillation techniques are revealed in Chinese Patent Application Nos. CN1428329, CN1431190A, CN1569809A, CN1131660A. Due to the nature of the catalytic distillation process, the structure of the reactor is necessarily complex and the operating conditions are quite demanding. Consequently this process is difficult to be scaled up for industrial scale production of DMC.
Therefore, there is clear and continuing need for developing new processes to manufacture DMC.